The invention herein relates to processes for the formation of calcium silicate hydrate objects, such as blocks of thermal insulation.
Calcium silicate hydrate bodies such as thermal insulation blocks have been in use for many years. For a long time such blocks were reinforced by asbestos fibers and were formed by a process known as "post-autoclaving" in which the raw materials (generally lime, silica and asbestos fiber) were mixed in an aqueous medium and then poured into molds having the shape of the blocks to be formed. The slurries of the raw materials in the molds were then autoclaved for in situ formation of calcium silicate hydrate objects directly in their desired form. This type of process was possible because the asbestos reinforcing fiber was not significantly degraded by the high temperature and high alkalinity conditions in the autoclave. Typical of such products was an asbestos-reinforced calcium silicate hydrate insulation sold commercially by the Johns-Manville Corporation under the trade name THERMOBESTOS.
Several years ago calcium silicate hydrate insulations appeared which were reinforced by materials other than asbestos, notably glass fibers and fibers of organic polymers such as polyesters. Because the glass and polymer fibers were degraded by the temperature and alkalinity conditions of autoclaving, it became a practice to form calcium silicate hydrate insulations by a process known as "pre-autoclaving." In this process the calcareous and siliceous raw materials of the calcium silicate hydrate are slurried in an aqueous medium and autoclaved to form fragile agglomerates of calcium silicate hydrate crystals. Following formation of the agglomerates the slurry is removed from the reaction vessel and the fibrous reinforcement is added to the slurry. The slurry of fibers and agglomerates is then passed to molding devices where the fiber reinforced calcium silicate hydrate insulation blocks are formed by pressure or vacuum molding to compress and interlock the agglomerates with each other. A typical description of such a process and the products formed therefrom will be found in U.S. Pat. Nos. 3,501,324 and 3,679,446. Materials of this type have been made and sold commercially by the Johns-Manville Corporation under the trademark THERMO-12. Other patents describing the hydrothermal formation of the calcium silicate hydrates and various aspects of the processing thereof include U.S. Pat. No. Re. 19,005 and U.S. Pat. Nos. 2,215,891; 2,665,996; 2,699,097; 3,116,158 and 3,816,149.
A particularly critical phase of a "pre-autoclaving" process occurs when the calcium silicate hydrate crystal agglomerate formation has been completed but the reaction slurry is still being held under high pressure and temperature in the reaction vessel. In order to be able to make the slurry available to the molders the temperature and pressure must be reduced. Pressure is normally reduced to ambient pressure since molders generally operate from reservoir tanks of slurry which are open to the atmosphere. Temperature must also be reduced to a level at which there is no significant tendency for the slurry to boil at ambient pressure or otherwise exhibit detrimental temperature effects. Originally pressure and temperature were reduced by simply venting the high pressure steam in the reaction vessel to the atmosphere. This was found to be extremely wasteful of thermal energy and in addition generally caused the reaction slurry to boil violently and break up the fragile agglomerates, thus severely degrading the critical crystal structure of the calcium silicate hydrate to the extent that the product did not mold satisfactorily. These problems were essentially entirely overcome by the development of the process described in aforesaid U.S. Pat. No. 3,816,149 issued to Joseph H. Zettel. In the Zettel process cooling water is injected into the slurry while the latter is at elevated temperature and pressure in the reaction vessel. The cooling water condenses the steam, lowering the temperature and pressure, and simultaneously dilutes the slurry to the concentration later used for molding. Condensation of the steam rather than venting eliminates boiling and fracture of the agglomerates, thus resulting in high yields of readily moldable calcium silicate hydrate crystals. Thermal energy is also conserved by being used to heat the cooling water and slurry mixture to the proper temperature for molding. With these distinct advantages the Zettel process has been used successfully in calcium silicate hydrate plants for several years.
The Zettel process has one significant drawback, however. Because the quantity of cooling water required to quench the reactor and condense the steam to "approximately ambient pressure" (as defined in the Zettel patent) is roughly equal in volume to the amount of aqueous slurry originally in the reactor, the reactor can only be filled about half full initially. Thus, during the entire reaction period the reaction vessel is only half full of material. Obviously if the vessel could be substantially completely filled initially with raw material, twice as much calcium silicate hydrate could be formed during each run period, in effect doubling production capacity of a given production unit without any increase in the number or size of reactors.
It is therefore an object of this invention to provide a calcium silicate hydrate reaction process which permits maximum utilization of the reaction process equipment.
It is also an object of this invention to provide a calcium silicate hydrate reaction process which significantly increases the production rate of calcium silicate hydrate as compared to the prior art processes.
It is also an object of this invention to provide a calcium silicate hydrate reaction process which provides significant conservation of thermal energy used in the process.